Geometry: What is it and why is it so important to buy a new bike?

Geometry is not exactly the first thing that comes to mind when choosing a bike, but it is one of the most important aspects that we should pay attention to, as it influences its responsiveness, comfort and agility. The angle of the head tube, the length of the top tube, the height of the bottom bracket, the reach and stack... all of these measures affect how the body weight is distributed, how the bike responds when cornering, how well it maintains balance at high speeds, and how comfortable and smooth it is to ride.

When we look at a bike geometry chart, we see a lot of numbers that tell you what a bike's design looks like and what its handling characteristics are. This blog covers the basic principles of road bike geometry.

Looking for a new bike? Then read this blog carefully to understand the importance of a bike fit before buying a new bike.

All aspects mentioned below are covered during a bike fit that specifically focuses on this. Using a software package, the bike can be fully designed and then transferred to the Purley Custom Fitbike in my studio. This way, the posture of different bikes and sizes can be compared and tested. This also covers all further aspects, such as the optimal length and angle of the stem, the ideal steering width and the most suitable crank length.

Let's take a look at all of this in more detail:

Head tube angle/Head tube angle

This is the inclination of the head tube, the one that connects the steering wheel to the fork. The wider this angle is, the faster and lighter the bike will be at the front, but also more unstable and uncomfortable to ride. A wider angle is perfect for riding on technical terrain and during races. By contrast, the narrower the angle, the more comfortable and stable the bike will be, but not as agile and reactive. A narrow angle is ideal for a much more relaxed form of cycling.

Road bikes usually have an angle of around 70-73º to allow the cyclist to have good control of the steering wheel, to gain speed easily and to climb smoothly. On the other hand, downhill bikes have angles around 63º to ensure stability during descents and technical trails, to prevent them from falling over the wheel.

Head tube length/Head tube length

‍‍The length of the previously mentioned head tube will strongly influence the measurements for the stack, which we will mention later. A longer head tube results in a higher steering position and, as a result, a more relaxed position on the bike. This is a pretty prominent feature in endurance road bikes. Time trial bikes, however, have significantly smaller tubes to improve aerodynamics.

‍Trail and Fork Offset ‍

The measurements in these two areas have a strong correlation:

- The trail is the distance between the steering axle and the point where the front wheel touches the ground. This distance is determined by the angle of the head tube and its length.

- The fork offset is the distance between the center of the front wheel and the steering axle. It can be used to reduce or increase trail distance without making changes to the steering axle angle. It can also be used to adjust the wheelbase distance (distance between both wheel axles). An increased offset means a reduced trail, and vice versa. On road bikes, you often see that the front fork is slightly curved to move the front wheel axle further forward.

Both measurements affect the bike's response and agility. A wider trail implies a more stable bike at high speeds but less easy to maneuver when driving slower. In contrast, a shorter trail distance implies a more agile bike and easier to maneuver at the front, but it can also make it more unstable at higher speeds.

‍Top tube length/Top tube length

This length is determined by the distance between the head tube and the seat tube. There are two different ways of considering this distance: one is the actual top tube, which refers to the length of the top tube itself, and then what's known as the effective top tube length, which is the horizontal line that runs from the top of the head tube to the top of the seat tube. The first was the most used before the rise of angled top tubes. Today, the second is more commonly used.

Generally, a bike with a shorter top tube will be more agile and easier to maneuver, and a bike with a longer top tube will be more stable and comfortable for longer rides.

The use of top tube length as an indicator of sitting position is somewhat outdated and has been replaced by Stack (+) and Reach (+). This is because most modern bikes rarely have a horizontal top tube. A more effective measure would therefore be the virtual top tube length, i.e. that of an imaginary horizontal top tube. As illustrated here in the picture.

Stack and Reach

The Stack refers to the height of the bike's frame and is measured from the center of the bottom bracket to the top of the head tube. This is the actual height of the frame and while a higher stack means a straighter position on the bike, a lower stack means a much more aerodynamic position.‍‍

As we mentioned earlier when talking about the length of the head tube, the Stack can also be raised by using spacers under the stem, or lowered by lowering the stem.

The Reach is the horizontal length from the center of the bottom bracket (in a vertical line) to the top of the head tube. This measurement influences the distance between the saddle and the steering wheel, and thus the rider's position on the bike. Generally, a longer Reach means a more extended or lowered position on the bike, while a shorter Reach means a straighter and firmer posture.

Stack and Reach can give you a good indication of your riding position, but it's not always that easy. Reach is useful when comparing similar bikes (e.g. two road bikes). Many modern bikes have longer Reach but shorter stems. This gives more control over the front of the bike and is popular with mountain bikes and gravel bikes. A longer Reach also means that the wheelbase is longer. This makes the bike feel more stable at high speeds.

A professional bike measurement will tell you what your optimal Stack and Reach values are, or you can use the measurements from your previous bike and adjust them if necessary. While searching for a new bike, look for Stack and Reach measurements that are as close to your desired numbers as possible.

Stack (+) and Reach (+)

These terms are increasingly being used by several bicycle manufacturers. These two measures provide a clearer picture of the totality of the geometry.

Stack (+) is the vertical height between the bottom bracket and the top of the steering wheel (where the stem clamps the steering wheel).

Reach (+) is the horizontal distance from the bottom bracket to the center of the steering wheel (where the stem clamps the steering wheel).

These measurements take into account the included spacers under the stem, different stem lengths combined with their angle, and cockpits. It shows where your steering position will be in relation to your bottom bracket.

Seat tube angle/Seat tube angle

This angle measures the inclination of the seat tube towards the ground. A wider angle (the seat tube then moves closer to the steering wheel) leads to an agile and efficient seat above the bottom bracket and a more forward-facing position on the bike, particularly effective on steep climbs. On the other hand, a smaller angle puts the cyclist more behind the bottom bracket, ideal for more comfort over long distances. In addition, it guarantees greater stability and control during descents.

Wheelbase/Wheelbase

This refers to the distance between the front and rear axles. It mainly influences the stability and maneuverability of the bike. A shorter distance leads to an agile and easy to maneuver bike, but is also exposed to the risk of instability at higher speeds. A wider distance between axles ensures greater comfort and stability, which can be beneficial for speed and better control on uneven terrain, although it can also be more difficult to handle. If you are looking for more driving comfort, you will find that endurance frames have the wheels further apart than with a frame focused on aerodynamics and speed.

Chainstay/Lying rear fork

This length refers to the distance between the rear wheel axle and the center of the crankshaft. A shorter chainstay guarantees better pedaling efficiency because it leads to better energy transmission. In addition, the rear wheel traction is improved thanks to more weight on it. This makes the bike more responsive, improves acceleration and makes it more agile and easier to handle on both climbs and flat sections.

On the other hand, a longer chainstay guarantees greater stability and control during descents, in addition to better shock absorption for greater comfort while driving.

Bottom bracket offset/Bottom bracket height

Sometimes referred to as “Bottom bracket drop,” this is a specific measurement for determining a bike's stability and primarily influences how well the bike responds when cornering. It is the distance between the horizontal line of the crankshaft and the horizontal line between the wheel hubs.

A larger offset means that the bike has a lower center of gravity. Cyclocross bikes, for example, have a lower offset than a road bike because of the obstacles they face on the course.

Conclusion

We can conclude that geometry plays a key role in how your bike handles and how it responds to your movements. We've described some basic concepts that cover all types of bikes, but keep in mind that different geometries exist within all the different modalities. In road cycling, some bikes are designed for competitions and others for a more relaxed form of cycling. In MTB, some bikes are specifically designed for downhill riding while others are more versatile, such as trail bikes.

Personally, I am of the opinion that it is always advisable to do a bike fit beforehand with a fit bike, as I described in the introduction. This way, you know in advance which bikes meet your personal requirements and you won't be surprised. Especially considering the current bikes, where the cabling runs inside and the steering wheel and stem form a whole (cockpit). This is labour-intensive to replace and also expensive.

With this in mind, the first thing to do is ask yourself what you're going to use your bike for, what types of terrain you plan to ride on, your own physical abilities, and your own personal preferences. The bikes that show the professionals on TV are focused on delivering performance. They mainly ride in a deep aggressive position to catch as little wind as possible. In addition, they train in this position every day and, in addition to cycling, they are also busy with agility and trunk stability exercises.

When choosing the size of your bike, you should pay particular attention to the Stack and Reach (better still Stack (+) and Reach (+). The rest of the bike's geometry measurements come second when choosing a bike that meets your needs and preferences.
In general, a bicycle's geometry must be sufficiently balanced to ensure a good combination of stability, ease of handling and comfort, bearing in mind that some disciplines require a more specific geometry. It's practically impossible to find the perfect bike for all terrains and all conditions.